The study of alteration of lomonosovite and vuonnemite: experiments in distilled water

Lykova I.S.*, Pekov I.V.*^,**, Yapaskurt V.O.*, Chukanov N.V.***

*Faculty of Geology, Moscow State University, Moscow, Russia; **Vernadsky Institute of Geochemistry and Analytical Chemistry RAS, Moscow, Russia; ***Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia.

The heterophyllosilicates lomonosovite, Na₁₀Ti₄(Si₂O₇)₂(PO₄)₂O₄,and vuonnemite, Na₁₁TiNb₂(Si₂O₇)₂(PO₄)₂O₃(F,OH), are the typical minerals of the hyperagpaitic alkaline rocks. The base of its crystal structure is the packets consisting of three layers. The central layer is formed by densely packed Na and Ti(Nb) octahedra. This layer is sandviched between two identical layers consisting of alternating the [Si₂O₇] groups and Ti octahedra. The Na₃PO₄ groups are located in the space between these packets.

According to Borneman-Starynkevich (1946) and Khomyakov (1976), lomonosovite and vuonnemite loose Na₃PO₄ and absorb H₂O under low-temperature hydrothermal conditions, transforming to murmanite and epistolite, respectively. Selivanova et al. (2008) performed similar experiments in the 1M solution of CaCl₂+NH₄Cl. According to their data, lomonosovite alters to beta-lomonosovite and vuonnemite alters to a sodium-deficient phase keeping phosphorus, without crystal structure changes.

In our experiments, grains (0.5–1.5 mm) of lomonosovite from Mt. Koashva, Khibiny alkaline complex, and vounnemite from the Palitra pegmatite, Mt. Kedykverpakhk, Lovozero alkaline complex (both – Kola Peninsula, Russia) were placed in distilled water at 90ºC and 1 atm and at room temperature for 570 hours for vuonnemite and 1000 hours for lomonosovite. The kinetics of processes was studied by conductometry. Samples after the experiments were examined by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), X-ray powder-diffraction (XRD) method and infrared spectroscopy (IRS).

The process of vuonnemite and lomonosovite alteration at the room temperature is very slow. No essential changes in the chemical composition, X-ray powder patterns and IR spectra are observed. Unlike, in products of the experiments carried out at 90ºC, SEM, EMPA and XRD show newly formed crystalline phases. Two consequently formed phases are detected in the product of vuonnemite alteration. “Typical” vuonnemte contains 10.5 atoms per formula unit (apfu; calculated on 4 Si atoms) Na and 2.0 apfu P, the 001 reflection of its XRD pattern is about 14.6 Å; the phase formed on the first stage of its alteration contains 6.1-6.5 apfu Na and 1.2-1.4 5 apfu P, its 001 reflection is 16.8 Å; the second phase, replacing the first phase, contains 4.4-5.3 5 apfu Na and 0.7-0.8 5 apfu P, its 001 reflection is 12.9 Å. One new sodium- and phosphorus-deficient phase was formed in the result of lomonosovite alteration. Its composition is very variable: 3.0-5.4 apfu Na and 0.6-1.1 apfu P. The 001 reflection is 12.7 Å. All the above-mentioned phases are not found in nature. Perhaps, they are relatively stable, intermediate phases which can be transformed to epistolite and murmanite, probably the most stable products of sodium phosphate leaching with hydration of vuonnemite and epistolite, respectively, for geological time

This work was supported by grants nos. 09-05-00143-a and 09-05-12001-ofi_m of RFBR and grant of President of Russian Federation no. NSh-3848.2010.5.

References:

Borneman-Starynkevich I.D. On chemical nature of murmanite // Problems of Mineralogy, Geochemistry and Petrography. AN USSR Publishing, Moscow-Leningrad. 1946. P. 66-74 (in Russian).

Selivanova E.A., Yakovenchuk V.N., Pakhomovsky Ya.A., Ivanyuk G.Yu. Features of low-temperature alteration of Ti- and Nb-phyllosilicates under laboratory condition // Minerals as Advanced Materials I. Springer Verlag, Berlin. 2008. P. 143-151.

Khomyakov A.P. Constitution and typochemical features of lomonosovite group minerals // Constitution and Properties of Minerals. Naukova Dumka Publishing, Kiev. 1976. Vol. 10. P. 96-100 (in Russian).